The present invention relates to an optical pickup installed in an optical disc driver, a magneto-optical disc driver, etc., for reproducing/recording optical information, and more particularly to an optical pickup constituting a subminiature and very light optical system using an aspherical single lens and a hologram.
In current systems for reproducing/recording optical information, there are several disc devices which are constructed in such a way that a disc-shaped optical information medium or a magneto-optical disc is rotated at a certain linear velocity and an optical pickup is provided to focus and scan light beams on the disc.
In these disc devices, an optical pickup is mounted with a delivery apparatus to be reciprocated along a radius or a circle within the radius range of the disc so as to scan light beams to the desired position on a disc. The optical pickup is also mounted jointly with an actuator to control all of an optical system or part of an optical system, including an object lens to the vertical and horizontal directions, for servo control of the light beam's exact focusing position within an accesses track, i.e., the focusing/tracking servo. Accordingly, when the optical pickup is manufactured, in order to achieve high accessing speed and a high degree of focusing and tracking control, the weight of the operative parts positioned by the delivery apparatus or an actuator should be reduced as much as possible.
The scope of optical pickups includes united optical pickups whose operative elements control the whole optical system, and separated optical pickups which control only parts of the optical system. An advantage of the united optical pickup is in the focusing/tracking servo because the optical axis of light source always equals that of the objective lens, however, the access time suffers due to its overall control of the system. Thus, in disc devices requiring high speed access, e.g., a magneto-optical disc device, separated optical pickups are typically used. However, since in the separated optical system the optical axis of the light source does not always equal that of the objective lens due to the focusing position of the light beam, errors are frequently generated in focusing error and tracking error detection. Moreover, control system of the separated optical pickup uses complicated logic to compensate for errors which result from severe influence due to disc vibration. Accordingly, high speed access and accurate servo operation is preferred so that the pickup can be moved fast.
However, the conventional optical pickup is composed of a series of lens groups for forming a parallel beam out of the diverging light from a light source, focusing and scanning it on the disc, and separating the light reflected from the disc from the incident light to ultimately be focused on a photodetector, creating limitations in size and weight. Moreover, conventional manufacturing is very difficult, because high accuracy is required, i.e., in units of micrometers.
Meanwhile, as a relatively simple structure, a pickup can be constructed by applying a hologram having an interference pattern of holography light as an object lens (or holo-lens). This uses the property wherein a spherical wave focused at one point from a reference light is diffracted by a function of the change in the wave surface of the hologram. But, when a parallel beam is maintained, moving only the holo-lens according to the up and down movement of the disc, the effective numerical aperture (NA) value is smaller. However, it is very difficult to converge the parallel beam, which complicates the constitution of the optical system except the holo-lens.